1. Field of the Invention
The present invention relates to an isolated communication system which transmits a digital input signal from a transmission unit to a receiving unit isolated from the transmission unit, and a transmission unit and a receiving unit which are applied to the isolated communication system.
2. Description of the Related Art
In regard to a device such as a switching power supply, or a motor driving device, which includes a high-voltage/high-current driving section such as a power device and a relatively low-voltage/low-current control section such as a controller for controlling (inverter controlling) of the driving section, there is known a configuration in which the driving section and the control section which have different breakdown voltages are electrically isolated from each other and transmission and reception of signals between the control section and the driving section are performed by isolated communication to prevent the control section from being broken down or operating abnormally due to high voltages and high currents used in the driving section. In such a device, the isolated communication between the control section and the driving section is performed in such a manner that a digital input signal input to a transmission unit of the control section is, after its conversion into a pulse signal, transmitted to a receiving unit of the driving section. In this communication, if some transmission errors occur due to disturbance or the like in a transmission path used for such isolated communication, it becomes unable to transmit correct signals. For example, in a case where pulses are generated according to detection of a rising and a falling of a digital input signal, if it cannot be identified correctly whether a pulse to be detected in the receiving unit is the rising or the falling of the digital input signal, decoding into a correct digital output signal in the receiving unit becomes impossible.
For the purpose of preventing such transmission errors, redundant signal lines or bits for correcting errors are generally added in digital communications or the like. This is not preferable, because its structure will become complicated and its cost will increase. Besides, there are increases in delay time and power consumption. In particular, if there is an increase in delay time, it becomes impossible to increase a frequency of switching. Therefore, performance of the device (for example, performance of power conversion) would be degraded significantly.
Also, there is known a configuration in which pulses respectively corresponding to a rising and a falling of a transmission signal are transmitted in different forms (for example, see U.S. Pat. No. 7,075,329 and JP-T-2001-513276). In U.S. Pat. No. 7,075,329, a pulse corresponding to a rising is a positive pulse while a pulse corresponding to a falling is a negative pulse. In JP-T-2001-513276, a single pulse is generated in correspondence to a falling while two pulses are generated in correspondence to a rising. Furthermore, there is known a configuration in which a pulse corresponding to a rising and a pulse corresponding to a falling are separately transmitted using different isolated communication paths (for example, see JP-T-2003-523147).